Methods for directional solidification casting

ABSTRACT

A method of directionally solidifying a molten alloy is presented. The molten alloy is disposed in a shell mold that has a thermal conductivity value greater than about 2 W/m-K. During the direction solidification, heat is transferred from the shell mold to a cooling region with a heat extraction rate greater than about 120 W/m 2 .

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with Government support under contract number DE-EE0005573, awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

BACKGROUND

The present disclosure generally relates to directional solidification, and more particularly, to directional solidification processes for casting large sized components at a large scale.

Directional solidification (DS) techniques enable the solidification of materials with grains aligned in a specific direction. Specifically, directional and single crystal (SC) structures are formed to improve the mechanical and metallurgical properties of the cast materials at their operating use temperatures. Various industrial DS/SC components of nickel-based alloys, iron-based alloys and cobalt-based alloys are traditionally produced by using the well-known Bridgman process. However, due to the thick ceramic molds and the poor cooling condition used in this process, it is difficult to maintain the required high temperature gradient necessary for the desired solidification structure.

Demand for a more efficient DS/SC casting with a high temperature gradient led to the development of modified techniques, such as liquid metal cooling (LMC). Although LMC results in higher cooling rates and finer microstructures compared to the conventional processes, it is still not widely applied in the industrial field due to the high equipment investments and lower throughput. Additionally, the heat transfer through the thick shell mold is not significantly improved by the LMC technique, this shortcoming is a barrier to further increases in thermal gradient, particularly in the casting of large-sized DS/SC components.

One way to improve throughput would be to increase the production rate at which components are being cast, which is typically limited by the rate of heat extraction, the rate of solidification of the alloy and the rate of withdrawal in the process. Because of this, it is a challenge to attain the correct microstructure during directional solidification and produce large castings with high production rate. Accordingly, there remains a need for improvements in directional solidification processes for large scale casting.

BRIEF DESCRIPTION

One embodiment of the invention is directed to a method for directionally solidifying a molten alloy disposed in a shell mold by transferring heat from the shell mold to a cooling region with a heat extraction rate greater than about 120 W/m².

Another embodiment of the invention relates to a method that includes directionally solidifying a molten alloy disposed in a shell mold, wherein the shell mold has a thermal conductivity value greater than about 2 W/m-K.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and aspects of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings.

FIG. 1 is a schematic diagram of a directional solidification casting apparatus, according to one embodiment of the invention;

FIG. 2 is a graphical representation of an improvement obtained in the coercivity of an ALNICO permanent magnet formed by using a directional solidification process according to an embodiment of the present invention;

FIG. 3 is a graphical representation of an improvement obtained in the remanence of an ALNICO permanent magnet formed by using a directional solidification process according to an embodiment of the present invention;

FIG. 4 is a graphical representation of an improvement obtained in the energy product of an ALNICO permanent magnet formed by using a directional solidification process according to an embodiment of the present invention.

DETAILED DESCRIPTION

Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary, without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.

As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances, the modified term may sometimes not be appropriate, capable, or suitable.

In the following specification and claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. The terms “comprising,” “including,” and “having” are intended to be inclusive, and mean that there may be additional elements other than the listed elements. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

The present invention relates to a method for manufacturing directionally solidified (DS) articles using, for example a Liquid Metal Cooling (LMC)-process. Aspects of the present invention can be employed to produce various castings from a wide variety of alloys including but not limited to nickel-based alloy, cobalt-based alloy, and iron-based alloy. An alloy to be cast is generally referred to as a casting alloy. While the advantages of the present invention are described with reference to the permanent magnets used in electric machines, for example ALNICO magnets, the teachings of the present invention are generally applicable to other components that may benefit from being unidirectionally cast.

A directionally solidified (DS) or single crystal (SC) casting is typically produced from a melt of a desired alloy often prepared by known vacuum induction melting techniques. As known in the art, heat transfer conditions during the solidification of the casting are controlled so that a solidification front advances unidirectionally and steadily to generate primary columnar crystals/grains, and to avoid the nucleation and formation of secondary grains from the melt in competition with the primary columnar single crystal. Moreover, the ability to control the crystal orientation of castings in the DS casting techniques proved to be highly beneficial in the casting of magnetic alloys, which results in improved magnetic properties along a certain direction in the cast product. The present invention proposes improved features and steps to enable directional solidification of large sized castings with improved mechanical and metallurgical properties (e.g., magnetic properties in the case of a magnetic alloy) for high throughput applications.

FIG. 1 schematically represents a casting apparatus 10 capable of producing a directionally solidified casting by an investment casting process. The casting apparatus 10 includes a high temperature region 12 (heating region) and a low temperature region 14 (cooling region). The heating region 12 usually includes heating elements 16. The cooling zone 14 is typically located directly beneath the heating region 12, while separated with the heating region 12 by a baffle or heat shield 18. The cooling region 14 generally includes a tank containing a liquid coolant 20. The coolant 20 typically includes a liquid metal bath such as, for example, molten tin at a temperature of about 250 degrees Celsius to about 500 degrees Celsius, or molten aluminum at a temperature of about 700 degrees Celsius to about 900 degrees Celsius. Other metals having low melting temperatures can potentially be used as a liquid coolant, such as lithium, magnesium, zinc, gallium, and indium.

The apparatus 10 further includes a shell mold 22 generally secured to a chill plate 24, and located within the heating region 12 to be heated to a temperature at least equal to, and typically above the liquidus temperature of a casting alloy. The shell mold 22, according to the aspects of the present invention, exhibits high thermal conductivity.

As known in the art, a shell mold is typically formed of a support having an outer surface and an inner surface that generally defines an internal mold cavity corresponding to the desired shape of a resulting casting or component. The support may include a fine particle dispersion reinforced with plurality of coarse ceramic particulates generally called “stucco”. Stucco may include a plurality of coarse ceramic particulates arranged in one or more layer form. A facecoat is generally disposed at the inner surface and a sealcoat is disposed at the outer surface of the support.

According to some embodiments of the invention, a high thermal conductivity stucco is used to build the shell mold 22. Any high thermal conductivity ceramic material may be used in the stucco to form a support of the shell mold 22. In one instance, the thermal conductivity of a material used as stucco in the support is greater than about 285 W/m-K. Non-limiting examples of high thermal conductivity stucco materials include materials such as silicon carbide, aluminum nitride, diamond, graphite, or any combination of the foregoing. In certain embodiments, the support includes silicon carbide stucco in addition to a fine particle dispersion of alumina and silica.

The high thermal conductivity stucco increases the overall thermal conductivity of the shell mold 22. Furthermore, a higher percentage of high thermal conductivity stucco material in the support may raise the thermal conductivity of the support, thereby allowing a thinner support and thus a thinner shell mold. In one embodiment, the stucco in the support is in a concentration greater than about 40 volume percent of the support. In one embodiment, the high thermal conductivity stucco is in excess of about 50 volume percent of the support. The shell mold 22, according to the aspects of the present invention, exhibits a high thermal conductivity. In one embodiment, the shell mold 22 has a thermal conductivity value, at a direction perpendicular to a surface of the mold 22, greater than about 2 W/m-K. In some embodiments, the thermal conductivity of the shell mold 22 is greater than about 5 W/m-K. In some embodiments, the thermal conductivity of the shell mold 22 ranges from about 5 W/m-K to about 15 W/m-K. In certain embodiments, the thermal conductivity of the shell mold 22 prepared using SiC stucco is greater than about 9 W/m-K. Various details of a high thermal conductivity shell mold and method of manufacturing the same are described in U.S. patent application Ser. No. 14/091,386 filed on Nov. 27, 2013.

In some embodiments, a method is provided for directionally solidifying a molten alloy (a casting alloy in molten form). In the first step of the method, the shell mold 22 that is placed within the heating region 12 is disposed with a molten alloy 26 to be cast. The molten alloy 26 may be poured into the shell mold 22 through a feed opening (not shown) at the top of the heating region 12. At the time of charging (for example, pouring), the shell mold 22 is fully within the heating region 12, for example a casting furnace. The shell mold 22 can be fully or partially filled with the molten alloy 26 depending on desired shape and size of the cast article. After filling the shell mold 22 with the molten alloy 26, the mold 22 is brought in contact with the liquid coolant 20 in the cooling region 14. The mold 22 is often withdrawn continuously from the heating region 12 to the cooling region 14 through a variable-sized opening 13 in the shield 18. The opening 13 enables the shield 22 to fit closely around the shape of the mold 22 as it is withdrawn from the heating region 12, through the heat shield 18, to the cooling region 14.

A direction of withdrawal 15 is indicated with an arrow. As soon as the shell mold 22 reaches the cooling region 14, solidification of the molten alloy 26 begins by transferring heat from the shell mold 22 to the coolant 20. A solidified alloy 28 (also referred to as a solidified casting) is found at a lower portion 23 of the shell mold 22, which already reached the cooling region 14. Approximately at the border between the heating region 12 and the cooling region 14 is a solidification front 30. A solidification front refers to a solid-liquid interface of the casting alloy during the solidification of the alloy. The solidification front 30 travels from the bottom of the shell mold 22 to the top in opposition to the direction of withdrawal 15.

Those familiar with the directional solidification understand the advantage of a high thermal gradient across the solidification front (solid-liquid interface) to yield good cast microstructure. In order to provide a high thermal gradient, heat needs to be removed quickly from the molten alloy. A thermal gradient at the solidification front may be given by equation (1).

$\begin{matrix} {G_{L} \approx \frac{{\frac{{CH}_{{eff}.{submerse}}}{d_{cast}}Q_{heat}} - {{\rho_{L}\left\lbrack {h_{f} + {C_{p.L}\left( {T_{melt} - T_{L}} \right)}} \right\rbrack}v}}{k_{L}}} & {{equation}\mspace{14mu} (1)} \end{matrix}$

Where

-   -   G_(L) is liquid thermal gradient at solidification front,     -   H_(eff, submerse) shell effective submersion in liquid coolant,     -   d_(cast) casting cross section equivalent diameter,     -   C constant determined by liquid metal (molten alloy) properties         and mixing conditions,     -   Q_(heat) heat extraction rate normal to shell surface,     -   ρ_(L) molten alloy density,     -   h_(f) casting heat of fusion,     -   c_(p,L) molten alloy heat capacity,     -   T_(melt) molten alloy temperature,     -   T_(L) liquidus temperature,     -   k_(L) molten alloy thermal conductivity,     -   v withdrawal rate.

Referring to FIG. 1, the temperature of the heating region 12 is typically maintained higher than a liquidus temperature of the casting alloy. A temperature difference between the temperature of a molten alloy (T_(melt)) in a casting furnace and the liquidus temperature (T_(L)) of the alloy is typically known as “superheat” of the alloy. A sufficient amount of superheat is typically required to achieve a highest possible thermal gradient (G_(L)) from the solid-liquid interface at the liquidus temperature. On the other hand, as evident from equation (1), a low superheat (T_(melt)−T_(L)) helps to enhance the thermal gradient at the solid-liquid interface. Some embodiments of the present invention include maintaining a superheat of the molten alloy 26 as low as possible while ensuring full development of thermal gradient. That is, the superheat may be maintained at a value equal to or slightly higher than a minimum superheat required for developing sufficient thermal gradient. A minimum value of the superheat may depend on various process parameters including the cooling profile, the withdrawal rate, and thickness of the baffle. In some embodiments, the superheat of the molten alloy 26 is maintained at least about 20 degrees Celsius, and more particularly at least about 30 degrees Celsius. In some embodiments, the superheat of the alloy 26 may be maintained in a range from about 30 degrees Celsius to about 50 degrees Celsius.

As mentioned previously, the baffle or heat shield 18 is usually placed between a bottom of the heating region 12 and the top of the cooling region 14 for shielding the cooling region 14 from the high temperature radiating from the heating region 12. The baffle typically includes one or more openings therein to permit passage of one or more molds there through as the molds are withdrawn from the heating region 12 into the liquid coolant bath 20. In some embodiments, the baffle 18 is a floating baffle that floats on the surface of the coolant 20 as shown in FIG. 1. Materials used for constructing the floating baffle 18 should be thermally insulating, chemically stable with respect to the liquid coolant 20, and have a density less than that of the liquid coolant to allow the baffle material to float on the coolant. Examples of ceramic materials suitable for use with liquid aluminum or tin include alumina and zirconia.

Various methods can be used for lowering the density of these materials or other chemically compatible materials. For example, hollow ceramic beads or bubbles may be formed of the desired material. Use of a ceramic bead baffle may help to promote a steep and steady thermal gradient during directional solidification casting normal to the solidification front 30, and to enable high withdrawal rates. The thickness of the ceramic bead insulation can be adjusted to maintain the position of the solid-liquid interface 30 above the level of the liquid coolant 20. In some instances, the thickness of the ceramic bead baffle may range from about 10 millimeters to 20 millimeters.

As understood by those skilled in the art and per equation (1), a higher heat extraction rate from a molten alloy enables faster solidification with a higher thermal gradient at the solidification front. A heat extraction rate is a rate at which heat is removed from a molten alloy to be solidified when the alloy comes in contact to a cooling region. Heat extraction from a solidifying alloy (i.e. molten alloy) in directional castings is generally limited by the heat transfer in the process, withdrawal rates, etc. According to the aspects of the invention, it was observed that the directional solidification of the molten alloy 26 disposed in the shell mold 22 occurred with a high heat extraction rate that is greater than about 120 W/m². Using shell mold 22 having high thermal conductivity (as discussed previously) greatly enhances the heat transfer across the shell mold 22 to the coolant 20, and increases the heat extraction rate as well as the solidification rate of the alloy. In some instances, the heat extraction rate during the solidification of the alloy 26 may be greater than about 150 W/m². In some instances, the heat extraction rate may range from about 150 W/m² to about 300 W/m².

It has been further observed that a high extraction rate may further improve the magnetic properties of a magnetic alloy when cast using direction solidification process as described herein. Effects of high extraction rate on magnetic properties of ALNICO alloy are described below with respect to an example.

An increased withdrawal rate of the shell mold 22 from the heating region 12 to the cooling region 14 adversely affects the thermal gradient (equation 1), however it is desirable to be high to improve the production rate. By achieving a high heat extraction rate, the withdrawal rate of the shell mold may be increased to a value that maintains a high enough thermal gradient to achieve desired microstructure and material properties and improves the production rate of castings. It has been observed that the withdrawal rate of the shell mold 22 during directional solidification casting can be increased greater than about 200 millimeters/hour. The withdrawal rate of the shell mold 22 can be increased to as high as 500 millimeters/hour, though in some instances, the withdrawal rate of the shell mold 22 may be maintained between about 50 millimeters/hour and 400 millimeters/hour to attain desirably high thermal gradient while maintaining high production rate for large scale castings. In certain embodiments, the withdrawal rate of the shell mold 22 may be maintained between about 100 millimeters/hour to about 300 millimeters/hour.

The present invention provides advantages over conventional directional solidification processes by enabling castings of large size articles with high throughput. Typically, small furnaces offer the advantage of high thermal gradient, allowing better foundry performance (microstructure, low porosity, absence of freckles, etc.), while in large size furnaces, for example industrial furnaces, the thermal gradient prevailing at the solidification front is low as well as difficult to measure. It has been a challenge to cast a large size article or component by directional solidification with high production rate to increase the throughput. Aspects of the present invention enable the use of a directional solidification process (for example LMC) to achieve and maintain high thermal gradients normal to the advancing solidification front, utilizing several features including high thermal conductivity shell mold and low superheat as described above, and thus allowing the casting of large parts as permitted by the overall dimensions of the casting unit with high throughput.

Example

The following example is presented for illustrative purposes only, and is not intended to limit the scope of the invention.

A shell mold was prepared by using a high emissivity facecoat having about 89.48 weight percent alumina (Al₂O₃), 2.24 weight percent chromium oxide (Cr₂O₃), and 1.78 weight percent titanium oxide (TiO₂) (all <50 microns) in a 6.5 weight percent colloidal silica (SiO₂) binder. A support part was formed by dipping a fugitive pattern into alumina slurry reinforced with high volume fraction of silicon carbide (SiC) stucco. Each coat of slurry and stucco were air-dried before subsequent coats are applied. The steps of dipping the pattern and drying were repeated until the desired thickness of about 5 millimeters to about 6 millimeters (mm) of the shell mold was obtained. Finally a thin layer of dark colored high emissivity SiC sealcoat was applied to the outer surface of the mold. After air drying, and dewaxing, the shell mold was then heated to a temperature of about 1000 degrees Celsius. The shell mold was subjected to an additional firing from about 1480 degrees Celsius to 1550 degrees Celsius in hydrogen for one hour.

The shell mold thermal conductivity, measured using a Synthetic Thermal Time-Of-Flight Imaging (STTOF) method, was about 9.3 W/m-K, which is a 540% improvement in comparison with the conventional alumina shell molds for the DS metal as shown in Table 1. The prepared SiC shell molds of about 6 mm in thickness were applied to make directionally solidified Fe-based alloy castings using liquid metal cooling at withdrawal rates ranging from about 100 millimeters/hour to 300 millimeters/hour.

TABLE 1 Shell mold Shell mold Thermal conductivity specimen wall thickness K (W/m-K) SiC-01 0.155 9.35 SiC-02 0.149 9.12 SiC-03 0.17 9.46 Mean 9.31 Standard 0.17 Alumina-1 0.35 1.94 Alumina-2 0.34 1.94 Almina-3 0.35 1.84 Mean 1.91 Standard 0.06

A shell mold made by using the above example was used for LMC directional solidification processes for improved crystalline anisotropy (reduced crystalline disorientation) and refined as-cast structure of ALNICO permanent magnets.

When liquid metal cooling is applied for directional solidification, heat extraction from solidifying casting is limited by shell thermal conductivity and shell thickness. A typical heat extraction rate for an alumina shell mold for LMC directional solidification was found to be about 112 W/m². Using the thin, and high thermal conductivity shell mold of the present example for LMC directional solidification process, the heat extraction rate of ALNICO processing was increased to about 240 W/m². The increased thermal gradient of the SiC shell mold provided fine microstructure of ALNICO magnets leading to high product yield and better product.

A high heat extraction of the shell mold helped in faster withdrawal of the structure thereby increasing the casting rate. Therefore, a DS casting of ALNICO magnet using the shell mold of the present example helped in faster ALNICO manufacturing production, including enabling a time and energy efficient post-cast heat treatment of the ALNICO alloy.

Furthermore, the high heat extraction rate in ALNICO alloy casting having about 7-12 weight percent Al, about 13-26 weight percent Ni, about 5-40 weight percent Co, up to about 6 weight percent Cu, up to about 12 weight percent Ti, with the balance Fe, was shown to have a significant effect on the demagnetizing properties of the ALNICO cast alloy as can be seen from the following example.

The effects of heat extraction rate on inherent coercivity (Hci), remanence (Br), and energy product (BH)max are depicted in FIG. 2, FIG. 3, and FIG. 4 respectively showing an experimental casting by using liquid metal cooling directional solidification according to the aspects of the present invention, as compared to a comparative casting by using conventional Bridgman directional solidification process.

The inherent coercivity of the experimental ALNICO casting made by LMC process is compared with that of the comparative casting made by Bridgman (BRG) process in FIG. 2 to demonstrate the effects of heat extraction rate on coercivity. Heat extraction rate in LMC ranges between 150 W/m² to 250 W/m², depending on the mold materials and shell thickness. Heat extraction rate in BRG process is about 60 W/m² to 80 W/m², depending on shell surface emissivity in vacuum. It was observed that with the increase in heat extraction rate, the inherent coercivity also increases.

Further, it was observed from FIG. 3 that with the increase in heat extraction rate, the inherent remanence increases, and FIG. 4 shows the increase in energy product with the increase in heat extraction rate. Hence it can be seen that as the thermal gradient increases the remanence and coercivity of the ALNICO magnet increases, thereby increasing the energy product.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 

1. A method comprising directionally solidifying a molten alloy disposed in a shell mold by transferring heat from the shell mold to a cooling region with a heat extraction rate greater than about 120 W/m².
 2. The method of claim 1, wherein directionally solidifying the molten alloy comprises the steps of; disposing the molten alloy into the shell mold; and contacting the shell mold containing the alloy with the cooling region.
 3. The method of claim 2, wherein contacting the shell mold with the cooling region comprises withdrawing the shell mold from a heating region into the cooling region.
 4. The method of claim 3, wherein withdrawing the shell mold comprises withdrawing the shell mold with a withdrawal rate ranging from about 50 millimeters/hour to about 400 millimeters/hour.
 5. The method of claim 3, wherein withdrawing the shell mold comprises withdrawing the shell mold with a withdrawal rate ranging from about 200 millimeters/hour to about 300 millimeters/hour.
 6. The method of claim 1, wherein directionally solidifying the molten alloy comprises maintaining a superheat of the molten alloy at least about 20 degrees Celsius.
 7. The method of claim 1, wherein the shell mold has a thermal conductivity value greater than about 2 W/m-K.
 8. The method of claim 1, wherein the shell mold has a thermal conductivity value in a range from about 5 W/m-K to about 15 W/m-K.
 9. The method of claim 1, wherein transferring heat from the shell mold to the cooling region comprises transferring heat with the heat extraction rate in a range from about 150 W/m² to about 300 W/m².
 10. The method of claim 1, wherein the alloy comprises a nickel-based alloy, a cobalt-based alloy, or an iron-based alloy.
 11. The method of claim 1, wherein the cooling region comprises a liquid coolant.
 12. A method comprising directionally solidifying a molten alloy disposed in a shell mold, wherein the shell mold has a thermal conductivity value greater than about 2 W/m-K.
 13. The method of claim 12, wherein the shell mold comprises a support having a face coat disposed on an inner surface of the support and a seal coat disposed on the outer surface of the support.
 14. The method of claim 13, wherein the support comprises a material having a thermal conductivity greater than about 285 W/m-K.
 15. The method of claim 14, wherein the material comprises silicon carbide, aluminum nitride, diamond, graphite, or a combination of any of the foregoing.
 16. The method of claim 12, wherein directionally solidifying the molten alloy comprises the steps of; disposing the molten alloy into the shell mold; and contacting the shell mold containing the molten alloy with a cooling region.
 17. The method of claim 16, wherein contacting the shell mold with the cooling region comprises withdrawing the shell mold from a heating region into the cooling region.
 18. The method of claim 17, wherein withdrawing the shell mold comprises withdrawing the shell mold with a withdrawal rate ranging from about 50 millimeters/hour to about 400 millimeters/hour.
 19. The method of claim 17, wherein withdrawing the shell mold comprises withdrawing the shell mold with a withdrawal rate ranging from about 100 millimeters/hour to about 300 millimeters/hour.
 20. The method of claim 16, wherein contacting the shell mold with the cooling region comprises transferring heat from the shell mold to the cooling region with a heat extraction rate greater than about 120 W/m².
 21. The method of claim 16, wherein contacting the shell mold with the cooling region comprises transferring heat from the shell mold to the cooling region with a heat extraction rate in a range from about 150 W/m² to about 300 W/m².
 22. The method of claim 12, wherein directionally solidifying the molten alloy comprises maintaining a superheat of the molten alloy at least about 20 degrees Celsius.
 23. The method of claim 12, wherein the shell mold has a thermal conductivity value in a range from about 5 W/m-K to about 15 W/m-K.
 24. The method of claim 12, wherein the alloy comprises a nickel-based, a cobalt-based, or an iron-based alloy. 